The present invention relates to a control system for controlling a support structure in a lithographic apparatus, a lithographic projection apparatus, a method of controlling a support structure in a lithographic apparatus, and a computer program product.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One of the most challenging requirements in micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of structures with respect to each other. For example, sub-100 nm lithography demands substrate-positioning and mask-positioning stages with dynamic accuracy and matching between machines in the order of 1 nm in 6 degrees of freedom (DOF), at velocities up to 3 m/s.
A widely used approach to achieve such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table), onto which is cascaded a fine positioning module. The coarse positioning module has a micrometer accuracy. The fine positioning module is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers. The coarse positioning module covers a large working range, while the fine positioning module only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in 6 DOF, large-range motions are rarely required for more than 2 DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy required for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schäiffel et al “Integrated electro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting, California, USA, 1996, p. 456-461. Similar encoders are also available commercially, e.g. position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH.
Position measurement for mask and substrate tables at the end of the fine positioning module, on the other hand, has to be performed in 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
As an alternative for interferometers it is known to use optical encoders, possibly in combination with interferometers. Such optical encoders are for instance disclosed in US 2004/0263846 A1, which document is hereby incorporated herein by reference. In US2006/0227309, optical encoders use a grid pattern on one or more grid plates to determine their position with respect to the grid pattern. The optical encoders are mounted on the substrate table, while the grid plate is mounted on a frame of the lithographic apparatus. Consequently, it is known where the substrate table is with respect to the grid plate.
With the continual desire to image ever smaller patterns to create devices with higher component densities, while keeping the number of patterns manufactured per unit time the same, or even increase that number, numerous tasks within the lithographic apparatus need to be performed faster. Consequently, accelerations and decelerations of the substrate table also increase, which may lead to vibrations. Due to the aforementioned vibrations, alignment becomes more difficult. Even though an alignment system and the grid plate may be coupled to the same frame within the lithographic apparatus, their relative position stability becomes insufficient to perform alignment of a substrate with respect to the substrate table at desired levels of accuracy.